1. Field of the Invention
This invention relates to a system and method of locating defects in a sample substrate by utilizing acoustic energy to generate a thermal signal that can be interpreted by an infrared detector and matching that thermal signal with a similar signal from a reference block applied directly to the sample.
2. Description of the Prior Art
In industries such as the turbine generator, aircraft, and many others, it is extremely important to maintain high standards of quality control because of safety concerns. It is important to be able to test machinery or aircraft parts for defects, such as pits, voids, corrosion, minor internal fractures and even service induced cracks, as in U.S. Pat. Specification No. 5,408,883 (Clark, Jr. et al.). There, a movable robotic diagnostic sampling device with ultrasonic and other type probes were used to monitor corrosion and potential cracks in heat exchanger tubes and the like in nuclear steam generators used in nuclear power plants. The sampling device could also cut a window in the tube wall and retrieve the sample window for mounting onto a separate tube for testing. The preferable way to monitor such problems, however, is on an assembly line before assembly and service. Preferably any tests would be non-invasive.
Much thought has been brought to the area of ultrasonic flaw detection and establishment of reference standards which contain flaws which potentially could be in the substrate to be tested. U.S. Pat. Specification No. 4,466,270 (Kimura et al.), relates to a reference block having an ultrasonic absorber attached to its back, for use in an ultrasonic examination method. This patent relates to a variation of a distance amplitude or area amplitude reference block used in pulse-echo ultrasonic flaw detection, where the reference discontinuity is a void within the reference block. When ultrasonic waves reach the back of the block they are partly transmitted to the absorber, where the ultrasonic energy is converted to heat energy and does not interfere with later, new ultrasonic pulses. This process results in decreased ghost interference on newer oscilloscope screens that use increased pulse repetition frequencies for automatic examination.
Other art in this area include, for example, U.S. Pat. Specification No. 4,729,235 (Podlech), related to a bonded specimen with intentionally placed, very small, discontinuities at the diffusion welded bond joint, that is also designed to provide reflectors in the path of ultrasonic waves which will produce the standard response on a time base ultrasonic sweep, and U.S. Pat. Specification No. 5,163,027 (Miller et al.) related to a standard calibration block incorporating a sequence of reflective interfaces designed to quantify the amplitude of the reflected ultrasonic energy with standard pulse-echo ultrasonic equipment. Also, U.S. Pat. Specification No. 5,837,880 (Shakinovsky et al.) which is a variation of the industry standard reference block, again, containing deliberately included features designed to be detected because they reflect ultrasonic energy back to a piezoelectric transducer. Reflectors are either voids or outer surfaces of the block.
None of this art appears to provide a thermal signal for detection by a thermal imaging camera. However, Thomas et al. in U.S. Pat. Specification No. 6,236,049B1 utilized an ultrasonic transducer coupled to an actual component, which might have a defect, through a malleable coupler, where the ultrasonic pulse energy from the transducer caused the defects, such as cracks, in the component to energize/heat up, such as by crack surfaces rubbing against each other so that thermal cameras could detect the defects or discontinuities from the heat, which appears as bright spot on the camera. Very short, even enclosed cracks as short as 0.75 mm are detectable by this method. This patent is an improvement of U.S. Pat. Specification No. 5,287,183 (Thomas et al.) which was an earlier system that was not as sensitive as the later Thomas et al. patent.
The component could be an in-use, or at rest aircraft fuselage, turbine blade component, rotor, vehicle cylinder head, or the like. The ultrasonic pulses used to heat cracks and defects in the 6,236,049B1 patent were simple pulses having substantially constant amplitude and did not need to employ sinusoidal signal modulation as used in vector lock in synchronous imaging.
This system allows detection of small cracks and tightly closed cracks. The problem with this system is, however, that it may not be particularly useful without reference standards or a means of characterizing defects appropriate reference standards designed specifically for this system.
Therefore a need has arisen to provide appropriate reference blocks/standards for use with a system that generates thermal signals for a thermal imaging camera from heat generated within a reference block/standard from ultrasonic pulse energy contact with a defect.